Robert Caro in his Manhattan office. The later volumes of his L.B.J. biography have taken more years to write than it took the former president to live them. ____________________

Robert Caro probably knows more about power, political power especially, than anyone who has never had some. He has never run for any sort of office himself and would probably have lost if he had. He’s a shy, soft-spoken man with old–fashioned manners and an old-fashioned New York accent (he says “toime” instead of “time” and “foine” instead of fine), so self-conscious that talking about himself makes him squint a little. The idea of power, or of powerful people, seems to repel him as much as it fascinates. And yet Caro has spent virtually his whole adult life studying power and what can be done with it, first in the case of Robert Moses, the great developer and urban planner, and then in the case of Lyndon Johnson, whose biography he has been writing for close to 40 years. Caro can tell you exactly how Moses heedlessly rammed the Cross Bronx Expressway through a middle-class neighborhood, displacing thousands of families, and exactly how Johnson stole the Texas Senate election of 1948, winning by 87 spurious votes. These stories still fill him with outrage but also with something like wonder, the two emotions that sustain him in what amounts to a solitary, Dickensian occupation with long hours and few holidays.

Caro is the last of the 19th-century biographers, the kind who believe that the life of a great or powerful man deserves not just a slim volume, or even a fat one, but a whole shelf full. He dresses every day in a jacket and tie and reports to a 22nd-floor office in a nondescript building near Columbus Circle, where his neighbors are lawyers or investment firms. His office looks as if it belongs to the kind of C.P.A. who still uses ledgers and a hand-cranked adding machine. There are an old wooden desk, wooden file cabinets and a maroon leather couch that never gets sat on. Here Caro writes the old-fashioned way: in longhand, on large legal pads.

Caro began “The Years of Lyndon Johnson,” his multivolume biography of the 36th president, in 1976, not long after finishing “The Power Broker,” his immense, Pulitzer Prize-winning biography of Moses, and figured he could do Johnson’s life in three volumes, which would take him six years or so. Next month, a fourth installment, “The Passage of Power,” will appear 10 years after the last, “Master of the Senate,” which came out 12 years after its predecessor, “Means of Ascent,” which in turn was published 8 years after the first book, “The Path to Power.” These are not ordinary-size volumes, either. “Means of Ascent,” at 500 pages or so, is the comparative shrimp of the bunch. “The Path to Power” is almost 900 pages long; “Master of the Senate” is close to 1,200, or nearly as long as the previous two combined. If you try to read or reread them all in just a couple weeks, as I foolishly did not long ago, you find yourself reluctant to put them down but also worried that your eyeballs may fall out.

The new book, an excerpt of which recently ran in The New Yorker, is 736 pages long and covers only about six years. It begins in 1958, with Johnson, so famously decisive and a man of action, dithering as he decides whether or not to run in the 1960 presidential election. The book then describes his loss to Kennedy on the first ballot at the Democratic convention and takes him through the miserable, humiliating years of his vice presidency before devoting almost half its length to the 47 days between Kennedy’s assassination in November 1963 (Caro’s account, told from Johnson’s point of view, is the most riveting ever) and the State of the Union address the following January — a period during which Johnson seizes the reins of power and, in breathtakingly short order, sets in motion much of the Great Society legislation.

In other words, Caro’s pace has slowed so that he is now spending more time writing the years of Lyndon Johnson than Johnson spent living them, and he isn’t close to being done yet. We have still to read about the election of 1964, the Bobby Baker and Walter Jenkins scandals, Vietnam and the decision not to run for a second term. The Johnson whom most of us remember (and many of us marched in the streets against) — the stubborn, scowling Johnson, with the big jowls, the drooping elephant ears and the gallbladder scar — is only just coming into view.

Johnson, who all along predicted an early end for himself, died at 64. Caro is already 76, in excellent health after a scary bout with pancreatitis in 2004. He says that the reason “The Passage of Power” took so long is that he was at the same time researching the rest of the story, and that he can wrap it all up, with reasonable dispatch, in just one more volume. That’s what he said the last time, after finishing “Master of the Senate.” (He also thought he could finish “The Power Broker” in nine months or so. It took him seven years, during which he and his wife, Ina, went broke.) Robert Gottlieb, who signed up Caro to do “The Years of Lyndon Johnson” when he was editor in chief of Knopf, has continued to edit all of Caro’s books, even after officially leaving the company (he also excerpted Volume 2 at The New Yorker when he was editor in chief there). Not long ago he said he told Caro: “Let’s look at this situation actuarially. I’m now 80, and you are 75. The actuarial odds are that if you take however many more years you’re going to take, I’m not going to be here.” Gottlieb added, “The truth is, Bob doesn’t really need me, but he thinks he does.”

In his years of working on Johnson, Robert Caro has come to know him better — or to understand him better — than Johnson knew or understood himself. He knows Johnson’s good side and his bad: how he became the youngest Senate majority leader in history and how, by whispering one thing in the ears of the Southern senators and another in Northern ears, he got the Civil Rights Act of 1957 through a Congress that had squelched every civil rights bill since 1875; how he fudged his war record and earned himself a medal by doing nothing more than taking a single plane ride; how, while vice president during the Cuban missile crisis, his hawkishness scared the daylights out of President Kennedy and his brother Robert. Caro has learned about Johnson’s rages, his ruthlessness, his lies, his bribes, his insecurities, his wheedling, his groveling, his bluster, his sycophancy, his charm, his kindness, his streak of compassion, his friends, his enemies, his girlfriends, his gofers and bagmen, his table manners, his drinking habits, even his nickname for his penis: not Johnson, but Jumbo.

This kind of knowledge does not come easily or cheaply. Caro has taken so long with Johnson that his agent, Lynn Nesbit, no longer remembers how many times she has renegotiated his contract; his publishing house has had two editors in chief, and no one there worries much about his deadlines any longer. The books come along when they come along. “I’m not a charity case,” Caro pointed out to me last month when I remarked on how Knopf had stuck by him all these years. It’s true that the Johnson volumes have been glowingly reviewed (“The Path to Power” and “Means of Ascent” both won the National Book Critics Circle Award and “Master of the Senate” won the Pulitzer Prize and the National Book Award) and that each of them has been a best seller, but it’s also true that they turn up so infrequently that Caro can hardly be thought of as a brand name. “Are the books profitable?” Sonny Mehta, Knopf’s current head, who took over the Johnson project — enthusiastically — after Gottlieb’s departure in 1987, said last month. He paused for a moment. “They will be,” he answered finally, “because there is nothing like them.”

Gottlieb is more philosophical. “So what if at the end of 45 years it turns out we lost money by one kind of accounting?” he said. “Think of what he has given us, what he has added. How do you weigh that?”

The two Bobs, Gottlieb and Caro, have an odd editorial relationship, almost as contentious as it is mutually admiring. They still debate, for example, or pretend to, how many words Gott&shy;lieb cut from “The Power Broker.” It was 350,000 — or the equivalent of two or three full-size books — and Caro still regrets nearly every one. “There were things cut out of ‘The Power Broker’ that should not have been cut out,” he said to me sadly one day, showing me his personal copy of the book, dog-eared and broken-backed, filled with underlining and corrections written in between the lines. Caro is a little like Balzac, who kept fussing over his books even after they were published.

Gottlieb and Caro also have slightly different accounts of how the Johnson project came about in the first place. Caro’s original contract called for him to write a biography of Fiorello LaGuardia, the former New York City mayor, after finishing Moses. Gottlieb says that in 1974, when Caro came in to talk about that project, he told him: “It’s a mistake. There were two gods in my house in the ’30s and ’40s: F.D.R. and LaGuardia. But LaGuardia is a dead end, an anomaly. He doesn’t come from anything, and nothing followed from him. I think you should write about Lyndon Johnson.” Turning to me and shaking his head he added: “You have to understand, I knew nothing about Lyndon Johnson and didn’t care about Lyndon Johnson, and it never crossed my mind until that moment that was what Bob should do. It was one of the inexplicable great moments, because it came out of nowhere.”

Caro says that he had already made up his mind that Johnson, who had only recently died, should be his next subject, partly because he didn’t want to write about New York again, but he listened quietly to Gottlieb. “I always felt that I increased my advance by a substantial amount by just sitting there not saying ‘That’s what I want to do,’ ” he told me.

Gottlieb and Caro argue about length, but they also argue about prose, even about punctuation. “You know that insane old expression, ‘The quality of his defect is the defect of his quality,’ or something like that?” Gottlieb asked me. “That’s really true of Bob. What makes him such a genius of research and reliability is that everything is of exactly the same importance to him. The smallest thing is as consequential as the biggest. A semicolon matters as much as, I don’t know, whether Johnson was gay. But unfortunately, when it comes to English, I have those tendencies, too, and we could go to war over a semicolon. That’s as important to me as who voted for what law.”

Their worst battle was over the second Johnson volume, “Means of Ascent,” which is largely about the stolen Senate election of 1948. Gottlieb encouraged Caro to tell this story at length because he was fascinated by the details of local politics, but he objected, as some reviewers did, to Caro’s characterization of Johnson’s opponent in that election, Coke Stevenson, a former Texas governor, who is painted in almost heroic terms. “We went mano a mano, chin to chin, nose to nose, I so disapproved of his idealization of Coke Stevenson,” Gottlieb said. “We just about killed each other.”

The editing of the most recent book went much more smoothly, Gottlieb said, explaining: “We both behaved better, and we really had a terrific time — maybe the first time we actually enjoyed the process. He could say, ‘I know you don’t want all this,’ and I could say, ‘How interesting that you know that!’ I think we have evolved, to the extent that we’re evolvable.” He laughed, and added: “How do these things happen? You just start in the belief that it’s all worth it, and before you know it, it’s 500 years later and you’re doing the notes on the 43rd volume.”

There was never a plan,” Caro said to me, explaining how he had become a historian and biographer. “There was just a series of mistakes.” Caro was born in October 1935 and grew up on Central Park West at 94th Street. His father, a businessman, spoke Yiddish as well as English, but he didn’t speak either very often. He was “very silent,” Caro said, and became more so after Caro’s mother died, after a long illness, when he was 12. “It was an unusual household in that I didn’t want to be there too much,” he said, adding that though he is fond of his younger sibling, Michael, now a retired real estate manager, they don’t have the kind of relationship that most brothers do. Caro spent as much time as he could at the Horace Mann School (it was his mother’s deathbed wish that he should go there) or else on a bench in Central Park with a book. He was always writing, and even then he wrote long. His sixth-grade essays dwarfed everyone else’s. His senior thesis at Princeton — on existentialism in Hemingway — was so long, he was told, that the college’s English department subsequently instituted a rule limiting the number of pages a senior could turn in.

Caro said he now thinks that Princeton, which he chose because of its parties, was one of his mistakes, and that he should have gone to Harvard. Princeton in the mid-’50s was hardly known for being hospitable toward Jews, and though Caro says he did not personally suffer from anti-Semitism, he saw plenty of students who did. “The way I thought of it, I wasn’t at Princeton,” he said. “I was at the newspaper and the literary magazine.” He had a sports column, “Ivy Inklings,” at The Daily Prince&shy;tonian, where he eventually became managing editor. (The top editor, until he flunked out, was R. W. Apple Jr., later to become a legendary New York Times reporter.) He also wrote short stories, or rather, not so short ones. One of them, about a boy who gets his girlfriend pregnant, took up almost an entire issue of The Princeton Tiger, a humor and literary magazine.

It was also at Princeton that Caro met his wife, Ina, who would also become the only assistant and researcher he has ever trusted. She was 16 at the time, a high-school student from nearby Trenton, double-dating at a Hillel mixer. She spotted Caro, very good-looking to judge from photographs taken around that time, across the room and announced to her best friend, “That’s the boy I’m going to marry.” Three years later, she did, dropping out of college against her parents’ wishes, and though she went on to finish her degree, get another one (in medieval European history) and write a couple of books of her own, she has to an extent remarkable by today’s standards devoted her life to his. At the lowest point during the writing of “The Power Broker,” when Caro had run out of money and was close to despair about being able to finish, she sold their house in suburban Long Island, moved the family (the Caros have a son, Chase, who is now in the information-technology business) to an apartment in the Bronx and took a job teaching school to keep him going.

“That was a bad time, a very bad time,” Caro recalled.

“I always felt that the most important thing was for Bob to be able to write,” Ina said. “Things like houses and money never meant much to me. I think they meant more to our dog,” she told me one morning in their big Upper West Side apartment, adding: “But I never thought this would be all he’d write about. I’ve always wanted him to finish a novel.” Even now, she went on, it’s hard for her to accept that Johnson will probably turn out to be the great work of their lives together. “You never think about dying,” she said. “You always think there’s going to be time.”

In order to marry, Caro needed a job. The Times offered him one as a copyboy for a salary that he now recalls as “something like $37.50 a week.” The New Brunswick Daily Home News and Sunday Times offered him $52 a week to be a reporter, and Caro took it. Another mistake, except that it led to an early lesson in power politics. The paper’s chief political writer was on leave to work for the Democratic Party in Middlesex County during an election. When he became ill, Caro took his place. He wrote speeches and did P.R. for one of the party bosses. On Election Day he rode around with this man to the polling places, and at one point they came upon the police loading some black people into a patrol wagon. “One of the cops explained that the black poll watchers had been giving them some trouble, but they had it under control,” Caro recalled. “I still think about it. It wasn’t the roughness of the police that made such an impression. It was the — meekness isn’t the right word — the acceptance of those people of what was happening. I just wanted to get out of that car, and as soon as he stopped, I did. He never called me again. He must have known how I felt.”

Caro had a further epiphany about power in the early ’60s. He had moved on to Newsday by then, where he discovered that he had a knack for investigative reporting, and was assigned to look into a plan by Robert Moses to build a bridge from Rye, N.Y., across Long Island Sound to Oyster Bay. “This was the world’s worst idea,” he told me. “The piers would have had to be so big that they’d disrupt the tides.” Caro wrote a series exposing the folly of this scheme, and it seemed to have persuaded just about everyone, including the governor, Nelson Rockefeller. But then, he recalled, he got a call from a friend in Albany saying, “Bob, I think you need to come up here.” Caro said: “I got there in time for a vote in the Assembly authorizing some preliminary step toward the bridge, and it passed by something like 138-4. That was one of the transformational moments of my life. I got in the car and drove home to Long Island, and I kept thinking to myself: ‘Everything you’ve been doing is baloney. You’ve been writing under the belief that power in a democracy comes from the ballot box. But here’s a guy who has never been elected to anything, who has enough power to turn the entire state around, and you don’t have the slightest idea how he got it.’ ”

The lesson was repeated in 1965, when Caro had a Nieman fellowship at Harvard and took a class in land use and urban planning. “They were talking one day about highways and where they got built,” he recalled, “and here were these mathematical formulas about traffic density and population density and so on, and all of a sudden I said to myself: ‘This is completely wrong. This isn’t why highways get built. Highways get built because Robert Moses wants them built there. If you don’t find out and explain to people where Robert Moses gets his power, then everything else you do is going to be dishonest.’ ”

Caro’s obsession with power explains a great deal about the nature of his work. For one thing, it accounts in large part for the size and scope of all his books, which Caro thinks of not as conventional biographies but as studies in the working of political power and how it affects both those who have it and those who don’t. Power, or Caro’s understanding of it, also underlies his conception of character and structure. In “The Power Broker,” it’s a drug that an insatiable Moses comes to require in larger and larger doses until it transforms him from an idealist into a monster devoid of human feeling, tearing down neighborhoods, flinging out roadways and plopping down bridges just for their own sake. Running through the Johnson books are what Caro calls “two threads, bright and dark”: the first is his naked, ruthless hunger for power — “power not to improve the lives of others, but to manipulate and dominate them, to bend them to his will” — and the other is the often compassionate use he made of that power. If Caro’s Moses is an operatic character — a city-transforming Faust — his Johnson is a Shakespearean one: Richard III, Lear, Iago and Cassio all rolled into one. You practically feel Caro’s gorge rise when he describes how awful Johnson was in college, wheeling and dealing, blackmailing fellow students and sucking up to the faculty, or when he describes the vicious negative campaign Johnson waged against Coke Stevenson. But then a volume later, describing Johnson’s championing of civil rights legislation, he seems to warm to his subject all over again.

In many ways, Caro’s notion of character is a romantic, idealistic one, and what fuels the books is disappointment and righteousness, almost like that of a lover betrayed. If there’s a downside to his method, it’s that anyone’s life, even yours or mine, described in Caro-esque detail, could take on epic, romantic proportions. The difference is that our lives would be epics of what it’s like not to have power, but the language would probably be the same. Caro has a bold, grand style — sometimes grandiose, his critics would say. It owes something to old-fashioned historians like Gibbon and Macaulay, even to Homer and Milton, and something to hard-hitting newspaperese. He loves epic catalogs (at the beginning of “The Power Broker” there is a long list of expressways that would not be out of place in the “Iliad” if only the Greeks and Trojans knew how to drive) and long, rolling periodic sentences, sometimes followed by emphatic, one-sentence paragraphs. He is not averse to repeating a theme or an image for dramatic effect.

This is not a style ideally suited to the chaste, narrow paragraphs of The New Yorker, especially in 1974, when it serialized “The Power Broker” in four installments that were long even then, when the magazine was so flush with ads it sometimes had trouble filling all its columns. I was a proofreader at The New Yorker then, and my office was across from that of William Whitworth, the editor of the “Power Broker” excerpts. I remember him wearily shuttling back and forth, like some Balkan diplomat, between the office of William Shawn, the magazine’s editor in chief, and one that Caro was borrowing while its occupant, Howard Moss, the poetry editor, was away for the summer. Caro complained that the magazine had tampered with his prose, and he wasn’t wrong. Instead of merely lifting some excerpts from the book manuscript, as was usually done, Whitworth tried to condense the whole thing, and this entailed squeezing out great chunks of writing, running the beginning of one paragraph into the end of another, pages away. “They softened my style,” Caro says. Shawn, on the other hand, had the magazine’s standards to uphold: The New Yorker insisted on its own, sometimes fussy way of punctuating; it didn’t approve of passages that were too leggy and indirect; it didn’t approve of repetitions; and it especially didn’t approve of one-&shy;sentence paragraphs. A description of the situation in vigorous Caro-ese might read something like this:

“In the editorial world, William Shawn was a man of immense power. He wielded it quietly, softly, almost in a whisper, but he wielded it nonetheless. Not for nothing did some of his staff members privately call him the Iron Mouse. For writers, Shawn’s long wooden desk was like a shrine, an altar, and in the passing of proofs across that brightly polished surface — pages and pages of proofs, stacks of proofs, sheaves and bundles of proofs, proofs from the fact-checkers, the lawyers, the grammarians, proofs marked with feathery hen-scratch and with bold red-pencilings — they discerned something like magic, the alchemy that renders ordinary, sublunary prose free of impurity and infuses it with an ineffable, entrancing glow, the sheen of true New Yorker style.

“But that style was not for everyone.

“It was not for Robert Caro.”

The negotiations became so fraught that between the second and third installments there was a weeklong gap, unthinkable in those days, while the two sides stared each other down and it seemed that the next two parts might be scuttled. Everyone at the magazine was aghast. Caro, it turned out, was as stubborn as Shawn. Here was a 38-year-old unknown who hadn’t published a word except in newspapers. Moreover, he was broke, hardly in a position to turn his back on the biggest payday of his life so far, but alone among New Yorker contributors at the time, he dared to become a Bartleby and turn his powerlessness into a point of principle.

Caro now says that Shawn agreed to restore all the changes he cared most deeply about, but the magazine version nevertheless differs from the original and changes Caro’s punctuation and paragraphing. The New Yorker series is a very readable redaction of the original — and without sacrificing much essential information, easier on the attention span than the book, which requires an immense time commitment — but for better or worse, it’s not as full-throated as the original.

Whitworth, undaunted, excerpted the first volume of the Johnson biography in The Atlantic after he became editor there in 1980.

It’s not writing that takes Caro so long but, rather, rewriting. In college he was such a quick and facile writer, and so speedy a typist, that one of his teachers, the critic R. P. Blackmur, once told him that he would never achieve anything until he learned to “stop thinking with his fingers,” and Caro actually tries to slow himself down these days. He doesn’t start typing — on an old Smith Corona Electra 210, not a computer — until he has finished four or five handwritten drafts. And then he rewrites the typescript. When I visited him one day in early December, he was correcting the page proofs of “The Passage of Power” the way Proust used to correct proofs: scratching out, writing in between the lines, pasting in additional sheets of inserts.

Caro is an equally obsessive researcher. Gott&shy;lieb likes to point to a passage fairly early in “The Power Broker” describing Moses’ parents one morning in their lodge at Camp Madison, a fresh-air charity they established for poor city kids, picking up The Times and reading that their son had been fined $22,000 for improprieties in a land takeover. “Oh, he never earned a dollar in his life, and now we’ll have to pay this,” Bella Moses says.

“How do you know that?” Gottlieb asked Caro. Caro explained that he tried to talk to all of the social workers who had worked at Camp Madison, and in the process he found one who had delivered the Moseses’ paper. “It was as if I had asked him, ‘How do you know it’s raining out?’ ” Gottlieb told me, and he added: “When ‘The Power Broker’ came out, other writers were amazed. No one had ever seen anything like it. It was a monument not to industry, because lots of people have industry, but to something else. I don’t even know what to call it.”

Caro once spent several nights alone in a sleeping bag in the Texas Hill Country so he could understand what rural isolation felt like there. For the Johnson books, he has conducted thousands of interviews, many with Johnson’s friends and contemporaries. (Lady Bird spoke to him several times and then abruptly stopped without giving a reason, and Bill Moyers, Johnson’s press secretary, has never consented to be interviewed, but most of Johnson’s closest cronies, including John Connally and George Christian, Johnson’s last press secretary, who spoke to Caro practically on his deathbed, have gone on the record.) He has spent literally several years at the Johnson Library, in Austin, Tex., painstakingly going through the red buckram boxes that contain Johnson’s papers, and he has been the first researcher to open some of the most revealing files there. “Over and over again, I’ve found crucial things that nobody knew about,” he said. “There’s always original stuff if you look hard enough.” He added that he tried to keep in mind something that his managing editor at Newsday, Alan Hathway, a crusty old newspaper&shy;man once told him, after pointing out that Caro was the only Ivy Leaguer who ever amounted to anything: “Turn every goddamn page.”

His notes, typed on long legal sheets, often with urgent directions to himself in capital letters, fill his cabinets, and before he begins writing, he indexes the relevant files in big loose-leaf notebooks that resemble the ones behind the counter at auto-parts stores. There is no computer, no Google, no Wikipedia.

One reason Caro’s books are so long is that he does keep burrowing through the files, and he keeps finding out things he hadn’t anticipated. Before beginning the first volume, he thought he could wrap up Johnson’s early life in a couple of chapters, until he talked to some of Johnson’s college classmates and found out about his lying, conniving side, which no one had previously described. That volume also includes a mini&shy;biography of Sam Rayburn, Johnson’s mentor in Congress, and a brilliantly evocative section about how electrification changed the lives of people in the Hill Country, much of it based on interviews conducted by Ina, who visited the women there with homemade preserves and eventually won them over, she says, because she was as shy and nervous as they were.

Caro thought that the 1948 Senate election would take up a single chapter or so in his Senate volume. Instead, it takes up most of a book of its own, what is now Volume 2. Johnson advocates used to say that “no one will ever know” whether that election was stolen. Caro knows, because he uncovered a handwritten memoir by Luis Salas, an election boss and party henchman, giving the details of how he falsified the records. The Senate book, Volume 3, begins with a 100-page history of the Senate, starting with Calhoun and Webster, because Caro felt that to understand the Senate you needed to see it in its great period. It includes minibiographies of Hubert Humphrey and Richard Russell Jr., the longtime Senate leader of the South, and ends with a detailed, almost vote-by-vote account of the passage of the Civil Rights Act of 1957. The first few weeks of the Johnson presidency, which take up so much of the new book, were originally imagined as just a chapter in what would be the final volume, and the new book also includes much more about the Kennedys than Caro anticipated. He goes into great detail, for example, about the feud between Johnson and Robert Kennedy, and the visits Bobby made to Johnson’s hotel room in Los Angeles after the Democratic convention in 1960, trying to talk Johnson into withdrawing from the vice-presidential nomination.

The installments keep ballooning, in other words, developing subplots and stories-within-the-story, in a way that reflects Caro’s own process of discovery. He is looking ahead to Volume 5 and to Vietnam, which is foreshadowed in the new book by Johnson’s hawkish impatience during the Cuban missile crisis. One day when I was visiting he pulled out a thick file of notes he had written, including transcripts, about the weekly Tuesday cabinet meetings Johnson had with Dean Rusk, Robert McNamara, Earle Wheeler and Walt Rostow, at which the question of whether to escalate was frequently discussed. “Look at this stuff,” Caro said to me. “It’s unbelievable!”

Caro now finds Johnson more fascinating than ever, he told me, and added: “It’s not a question of liking or disliking him. I’m trying to explain how political power worked in America in the second half of the 20th century, and here’s a guy who understood power and used it in a way that no one ever had. In the getting of that power he’s ruthless — ruthless to a degree that surprised even me, who thought he knew something about ruthlessness. But he also means it when he says that all his life he wanted to help poor people and people of color, and you see him using the ruthlessness, the savagery for wonderful ends. Does his character ever change? No. Are my feelings about Johnson mixed? They’ve always been mixed.”

On a corkboard covering the wall beside Caro’s desk, he keeps an outline, pinned up on legal-size sheets, of “The Years of Lyndon Johnson.” It’s not a classic outline, with indentations and numbered headings and subheadings, but a maze of sentences and paragraphs and notes to himself. These days, part of the top row is gone: the empty spaces are where the pages mapping the new book used to be. But there are several rows left to go, and 13 additional pages that won’t fit on the wall until yet more come down. Somewhere on those sheets, already written, is the very last line of “The Years of Lyndon Johnson,” whatever volume that turns out to be. I begged him more than once, but Caro wouldn’t tell me what that line says.

Caro has no shortage of plans for what to do next, after he finishes with Johnson, and he has already picked out a topic, though he won’t reveal what it is. He also told me he could imagine writing a biography of Al Smith, the New York governor and 1928 presidential candidate. But it’s also possible that at some level he doesn’t really want to be done — that without entirely intending to, he’s eking Johnson out — because whenever a biographer finishes, burying his subject, he dies a little death, too. Caro is a great student of Gibbon, and he must be familiar with what Gibbon wrote in his house at Lausanne, Switzerland, in 1787, after completing his “Decline and Fall of the Roman Empire”: “I will not dissemble the first emotions of joy on the recovery of my freedom, and perhaps the establishment of my fame. But my pride was soon humbled, and a sober melancholy was spread over my mind by the idea that I had taken an everlasting leave of an old and agreeable companion, and that, whatsoever might be the future fate of my history, the life of the historian must be short and precarious.”